The present invention broadly relates to an improved aqueous acidic zinc and alloy electroplating solution and process employing such solution for depositing a corrosion resistant and/or decorative zinc or zinc alloy plating on a variety of conductive substrates including ferrous substrates such as iron and steel. Such aqueous acidic zinc and zinc alloy electrolytes which may be of the chloride as well as the sulfate type in accordance with prior practice conventionally contain substantial amounts of boric acid as an essential ingredient which contributes to the buffering of the bath and also imparts beneficial characteristics to the zinc or zinc alloy electrodeposit. Typically, commercial acid zinc and zinc alloy baths contain a minimum of about 25 grams per liter (g/l) of boric acid and more typically, about 30 to about 35 g/l. The operating pH of such commercial zinc and zinc alloy baths typically ranges from about 4 to about 6 and it is conventional practice to employ soluble zinc anodes for replenishing the zinc ion concentration during an electroplating operation. In a zinc alloy electroplating bath, the alloying metal ions such as nickel and/or cobalt are conventionally replenished by the addition of bath soluble and compatible salts of such alloying ions.
A continuing problem associated with such aqueous acidic zinc and zinc alloy electroplating baths has been the formation of insoluble zinc polyborate compounds which form a coating on the zinc anodes as well as precipitates in the electroplating bath. The tendency toward the formation of such undesirable insoluble polyborate compounds is aggravated as the concentration of boric acid increases, during periods of quiescent standing of the bath such as over weekends and when the temperature of the bath decreases. It is reported that such polyborate compounds contain from about 3 to about 7 molecules of borate and are extremely insoluble such that the buildup of a coating of the polyborate compound on the zinc anodes drastically reduces the conductivity of the bath and the dissolution of the soluble zinc anodes necessitating frequent removal of the zinc anodes and a grinding or scraping of the surfaces thereof to restore the process to satisfactory commercial operation. The necessity of frequently cleaning the zinc anodes constitutes a time consuming, costly and tedious operation and in recognition of this problem, it has been proposed to eliminate boric acid as an ingredient in such acid zinc plating baths. It has been found, however, that the complete elimination of boric acid drastically reduces the range of usable current densities for achieving uniform commercially acceptable zinc or zinc alloy electrodeposits which has detracted from a widespread commercial acceptance of boric acid-free aqueous acidic zinc or zinc alloy electroplating solutions.
The foregoing problem is overcome in accordance with the improved aqueous acidic zinc and zinc alloy electrolyte and process of the present invention whereby the bath can operate at relatively low boric acid concentrations by virtue of the inclusion of a controlled amount of a polyhydroxy additive agent which substantially eliminates or drastically reduces the formation of the bath insoluble polyborate compounds significantly increasing the useful operating life of the bath and the zinc anodes while at the same time providing for decorative bright, ductile zinc and zinc alloy electrodeposits over a broad range of current densities.